1. Field of the Invention
This invention relates to a display device and, more particularly, to a spontaneous emission-type display device such as organic electroluminescense device.
2. Background Art
A device using electroluminescence of an organic material (hereinafter called “organic EL device”) is made of an organic layer in which an organic hole transport layer, or organic light emitting layer, interposed between a first electrode and a second electrode. This device is characterized as a spontaneous emission type display device capable of high-luminance emission by low-voltage d.c. driving.
FIG. 1 shows a cross-sectional configuration of a central part of a transmission-type organic EL device as one of such organic EL devices. The organic EL device shown in FIG. 1 is made by stacking on a transparent substrate 1, sequentially from bottom to top, a transparent electrode 2, organic buffer layer 3, organic hole transport layer 4, organic light emitting layer 5 and metal electrode 6, and light h generated in the organic light emitting layer 5 is extracted through the substrate 1.
In the organic EL device shown in FIG. 1, however, spectrums of respective colors in the light h generated in and extracted from the organic light emitting layer 5 having various emission colors have wide peak widths as shown in FIG. 2, and especially regarding the red light h, the peak wavelength is deviated to a lower wavelength. Therefore, with a display apparatus made by using such organic EL devices for color representation, it was impossible to obtain a sufficient color reproduction range enough to display, for example, TV images.
To overcome this problem, it was proposed to interpose a dielectric mirror layer (not shown) between the substrate 1 and the transparent electrode 2 such that the dielectric mirror layer, organic buffer layer e, organic hole transport layer 4, organic light emitting layer 5 and metal electrode 6 form a cavity structure. In the organic EL device having this cavity structure, light h generated in the organic light emitting layer 5 reciprocally moves between the dielectric mirror layer and the metal electrode 6, and only a part of light with the resonant wavelength is extracted through the substrate 1. Therefore, light h having a high peak intensity and a narrow spectrum can be extracted, and it is possible to enlarge the color reproduction range of a display apparatus using such organic EL devices.
However, as the peak width of the spectrum of extracted light h is narrowed, as it is in the organic EL device having the above-mentioned cavity structure, dependency of the emission property on the view angle increases. For example, when the emission surface is viewed from an aslant direction, wavelength of light h largely shifts and the emission intensity decreases. Therefore, it is important that the spectral width of light extracted from the organic EL device is not narrow too much. However, the organic EL device does not envisage the dependency of the view angle in its design, and could not maintain a sufficient color reproduction range over wide view angles.
Moreover, the organic EL device, in problem, had to optimize the cavity structure for light h of each color to be extracted, and required an additional effort for it.
Further, in the organic EL device shown in FIG. 1, because external light intruding from outside the device is reflected by the metal electrode 6, its external light reflectance is high and this results in a low contrast under external light. As a technique for preventing it, Japanese Patent Laid-Open Publication No. hei 9-127885 discloses an organic EL display apparatus having a combination of a ¼ wavelength panel and a linear polarizing panel disposed in front of the front surface, as shown in FIG. 3. More specifically, this organic EL display apparatus locates a combination of a ¼ wavelength panel 8 and a linear polarizing panel 9 in front of a substrate 1 in an organic EL device having the same configuration already explained with reference to FIG. 1 for the purpose of preventing reflection of external light. It is also proposed to replace the metal electrode behaving as a reflective surface with a transparent electrode and providing a light absorbing layer on side of the transparent electrode opposite from the organic layer such that the light absorbing layer absorbs external light and prevents reflection of external light. However, in those display apparatuses having those configurations, extraction and reflection of light generated in the display apparatus are also prevented, and their luminance decreases to about 50%.
It is also proposed to dispose color filters transmitting red (R), green (G) and blue (B) colors, respectively, on respective color emission pixels. Although display devices with this configuration could prevent reflection of external light other than emitted light, they cannot prevent reflection of external light outside the wavelength ranges of the emission colors of respective pixels.
It is therefore an object of the invention to provide a spontaneous emission type display device that can maintain a sufficient color reproduction range over a wide view angle.
Another object of the invention is to provide a spontaneous emission type display device that can reduce reflection of external light and thereby improves the contrast without inviting a decrease in luminance.
A further object of the invention is to provide a spontaneous emission type display device that can maintain a sufficient color reproduction range over a wide view angle and can improve the contrast by reducing reflection of external light without inviting a decrease in luminance.